Solidify modifier isn’t working as intended - it scales outwards instead of applying thickness
Modifiers act on the ‘base’ mesh and I believe it is acting on the mesh as expected but the problem is that you cannot see the effect of the solidify due to the scale of the mesh in one dimension.
In your example you are seeing the effect of thr modifier in the X and Y dimensions but not in Z -as you have scaled it to a size of zero in that direction to make it flat. This means that the solidify is also squashed flat so you see no effect.
You should select the mesh, press Ctrl+A and choose Apply Scale. This will reset the scaling of the mesh back to 1,1,1 and your Solidify should again behave as expected.